In the construction of pipe lines wherein elongated pipe sections are welded together by forming girth welds, such welds must be examined for defects utilizing, for example, ultrasonics and X-ray or gamma-ray radiography. However, while the use of such known techniques yield approximate characteristics of the weld defects, as derived from changes in densities of the pipe wall, such techniques do not yield sufficiently accurate data related to the geometry and depth of undercuts or cracks. Specifically, it is believed that such techniques are not sufficiently accurate for providing a precise depth determination of undercuts or toe cracks, as called for by the specifications of the American Petroleum Institute.
An example of such a weld defect, in the form of an undercut, is shown in FIG. 1 of the drawings. Referring specifically to FIG. 1, a cross-section of a butt weld is depicted wherein the ends of two pipe sections are welded together. Prior to effecting the weld a band is affixed to the pipe and a bevel machine is mounted on the band for circumferential movement to provide each pipe and with a beveled surface as indicated. Next, a welding machine may be mounted on the band to effect the weld illustrated in FIG. 1. Due to various causes, defects may occur in the weld, as also illustrated in FIG. 1, wherein an undercut may develop at an inner edge of the weld. While specifications defining permissible limits for such undercuts have been defined by the American Petroleum Institute, and while remedial procedures for such defects have also been specified, it is believed that the state of the art of measuring techniques has not been fully adequate to ensure adherence to such specifications.
For example, in one prior art proposal for inspecting welds, as disclosed in U.S. Pat. No. 3,213,676, a method is described for detecting voids internally of a weld, but the structure and process described in that process is incapable of defining the depth and profile of an undercut as required by the abovementioned specifications of the American Petroleum Institute.
Accordingly, a principal object of the present invention is to provide a method and apparatus for detecting the depth and circumferential extent of flaws in pipe line girth welds. The invention utilizes a novel process and apparatus for transmitting, receiving, and processing acoustic energy pulses in order to provide an accurate profile of weld defects.